A cholesteric LC can be obtained by incorporating a chiral additive into a nematic LC. In the cholesteric LC, the long axes of rod-like LC molecules twist around a helixcal axis sequentially, and thus the cholesteric LC has a helical structure. The chiral pitch, P, refers to the distance over which the LC molecules undergo a full 360° twist. P varies inversely proportionally with the amount of the chiral additive in a LC. A cholesteric LC can exhibit a special optical property of selective Bragg reflection due to the helical structure thereof. A cholesteric LC having a single pitch can reflect a light having a certain wavelength, satisfying with the equation: Δλ=ΔnP, wherein Δn represents the refractive index of birefringence. In a reflection wave range, the incident light at an optical rotary direction same as the twisting direction of the helical structure of the cholesteric LC, can be reflected; while the incident light, which has a inverse optical rotary direction with respect to the twisting direction of the helical structure of the cholesteric LC, can be transmitted. Therefore, a cholesteric LC is useful in various applications, such as a reflection-type polarizer having no loss of light, a brightness enhancement film for a LC display, IR ray shielding film material, and the like.
For visible lights, the reflection wavelength range of a cholesteric LC having a single pitch is less than 150 nm. It has been proved that the reflection wavelength range of a cholesteric LC can be efficiently extended by forming a gradually or heterogeneous pitch distribution in the cholesteric LC. Yuwen CHEN et. al reported in CN1549036A a method, comprising the step of creating a heterogeneous electrical field on an electrode, such that the pitch of the LC near to the electrical field increases and the pitch of the LC far from the electrical field is essentially unchanged, thereby the reflection wavelength range is extended. The defect of said method is that an electrode having a particular shape should be incorporated into a reflection film, which would affect the optical performance of the film. Also, such an electrode is difficult to be processed.
In addition, a macromolecule-stabilized LC, which is formed by stabilizing the crosslinked macromolecule network dispersed in the LC and fixing the alignment of the LC molecules, can provide a desirable macroscopic-alignment distribution of LC molecules. Such a macromolecule-stabilzed LC is a conventional means in the art to obtain a particular pitch distribution of a cholesteric LC. D. J. Broer et. al. (CN1198819A) described a method, including steps of providing a complex material system (comprising a light-polymerizable cholesteric alcohol acrylate, a light-polymerizable nematic alcohol acrylate, a dye) that is polymerizable upon light irradiation, and applying a weak UV irradiation (the luminous intensity is less than 0.05 mW/cm2) to the complex material system, such that the monomers in the system can be dispersed for a sufficient period to form a gradual pitch distribution, thereby a polarizer that can selectively reflect in the whole wavelength range of visible light is obtained. Since the dispersion of the monomer in the LC varies depending upon the intensity of the UV light, the wavelength range obtained through such a method is sensitive to the luminous intensity of the UV irradiation. Thus, during the production process of the polarizer by said method, the polarizer should be tested by a homogeneous light sensor. Once a desired wavelength range is achieved, the luminous intensity of the UV irradiation has to be increased immediately. Thus, a complex apparatus is needed for carrying out said method and the parameters of the process are difficult to be controlled.